Amplifiers are used in all cell phones to amplify a modulated signal from a transceiver prior to passing the signal through the front end of the cell phone, which typically consists of passive components and a switch into an antenna where the signal is transmitted to a base station. FIG. 1 illustrates a high level topography of the circuits contained in a prior art cell phone. Generally, a baseband integrated chip 1 provides a signal to a transceiver 2. The transceiver transmits an RF signal to a power amplifier 3, which outputs an RF signal to a front end module 4 and out into the atmosphere by an antenna 5. All of these components are housed within a typical cell phone 6.
Two important factors driving the design of RF/microwave power amplifier 3 are linearity and efficiency. As used herein, linearity refers to the device's ability to amplify without distortion, and efficiency refers to the device's ability to convert DC power to RF/microwave power with as little wasted energy as possible. In conventional power amplifier designs, improvement in one area typically causes degradation in the other. An RF/microwave amplifier has two regions of operation: linear and nonlinear. In the linear region, the input signal envelope is amplified and no distortion is present at the output. For large peak to peak input signal levels, the amplifier enters the non-linear region and the output signal becomes distorted.
Distortion in RF/microwave amplifiers is generally caused by amplitude clipping, phase variations as a function of signal amplitude, and intermodulation products. Amplitude clipping occurs when the peak to peak input signal envelope amplitude extends beyond the linear region of the amplifier. Phase variations with signal amplitude also result when the peak to peak input signal envelope amplitude extends beyond the linear region of the amplifier. Intermodulation distortion (IMD) occurs as a result of nonlinearities in the amplifier transfer function resulting in mixing products being generated at the sum and difference frequencies of the input signals.
The third order intermodulation product (IMD3) is of great interest since this product is very close to the carrier signal on the frequency spectrum. Being located so close to the carrier signal, IMD3 is very difficult to eliminate or even reduce and is often the limiting factor in the linearity of the RF/microwave amplifier. The output third-order intercept point (OIP3), also known as OTOI (Output Third Order Intercept), is defined as the intersection point between the extrapolated 1:1 slope of the fundamental output power and the extrapolated 3:1 slope of the third-order intermodulation products. If the extrapolations are done well within the linear region, the OIP3 (OTOI) becomes a useful specification for predicting the linearity of the power amplifier. Thus, the higher the OIP3 (OTOI) point, the more linear the power amplifier. As mentioned above, reducing the IMD of the power amplifier improves its linearity and thus improves the OIP3 (OTOI).
Referring to FIG. 2, power amplifier 3 comprises one or more stages 7a, 7b . . . 7n connected in series by matching networks 8a, 8b, 8c . . . 8n that consist of a circuit topology of zero or more resistors, capacitors, and inductors. Each stage 7a, 7b . . . 7n consists of a number of branches 18a, 18b . . . 18n connected in parallel. Each branch 18 has of one or more unit cells 20 connected in parallel. A unit cell is composed of one or more transistors in a circuit topology of zero or more resistors, capacitors, and inductors.
Each power amplifier stage 7 is typically biased with a bias circuit (not shown) that provides the appropriate current or voltage for the branches to operate in a single class of operation. As used herein, a class of operation is determined by the percentage of an input sinusoidal signal during which the unit cells in each branch is on and conducting current. For example, in Class A operation, the branches are all biased such that they are on and conducting current for 360 degrees of the input sinusoid signal. In Class B, the branches are on and conducting for 180 degrees of the input signal. The near-Class B biasing condition is for the case where the branches are on and the conduction angle is close to but above 180 degrees. In Class AB, the branches are on and conducting typically at or around 270 degrees but may vary between 180 degrees and 360 degrees. The limits for each class of operation are not rigorously set and are used herein for purposes of general understanding of the operating condition of an amplifier.
Several methods have been tried to improve the response of power amplifiers. One method is to operate the RF/microwave amplifier at lower power levels to ensure that the device remains in the linear region. A drawback to this method is that when the device is operated at a lower power level it is operating less efficiently than it does at higher power levels.
The present invention seeks to improve the output response of RF/microwave power amplifiers by reducing IM3 levels and improving the OIP3 (OTOI) of the power amplifier. One solution to this problem is to reduce the distortion present in the linear and nonlinear region so that the operation of the amplifier can be extended into the nonlinear region where higher power levels are possible.